In recent years, as a technology for increasing communication capacity without widening the occupied frequency band, MIMO communications systems that use multiple transmitting antennas and multiple receiving antennas have been put into use in wireless communications. This technology has been developed so far, mainly aiming at application to non-line-of-sight (NLOS) mobile communications systems, such as mobile phones, wireless LANs (Local Area Networks) and the like. However, recently, application of this technology to line-of-sight fixed wireless communications systems using microwaves and millimeter waves has been being investigated.
In line-of-sight fixed wireless systems, increase in communication capacity has been previously achieved in a limited bandwidth by means of ultra-multilevel modulation using increased number of modulation levels and polarization multiplexing using both horizontal and vertical polarizations. However, as a technology for meeting the demand for large communication capacity accompanied by increase in traffic, MIMO technology has drawn attention.
Driessen et al. describes the principle of MIMO transmission in line-of-sight (LOS-MIMO) fixed wireless communications [NPL 1]. Driessen et al. [NPL 1] and Sarris et al. [NPL 2] demonstrated that adjusting the difference between transmission delays by appropriate arrangement of multiple transmitting antennas and receiving antennas contributes to improvement of the signal-to-noise ratio thanks to the carrier's phase rotation resulting from the adjusted transmission delay difference to enable increase in communication capacity. In a case having two transmitting antennas and two receiving antennas, the signal-to-noise ratio is expected to improve by 3 dB at most with communication capacity doubled, compared to the ordinary single-input single-output (SISO) transmission having one transmitting antenna and one receiving antenna. The line-of-sight MIMO communications of this kind is distinguished from non-line-of-sight MIMO (NLOS-MIMO) communications in mobile communications, wireless LANs, etc. which positively and effectively use multiplexed transmission waves temporally varying due to superposition of many signals resulting from wave reflection, diffraction and dispersion.
Examples of the MIMO communications in such line-of-sight fixed wireless systems using the microwave bands and millimeter wave bands have been disclosed in [PTL 1], [PTL 2] and [PTL 3]. The systems disclosed in these references include a tandem configuration having a means for separating and extracting a desired signal from received signals that have been multiplexed by MIMO transmission and a means for performing an ordinary demodulating process for the extracted signal. The ordinary demodulating process means a demodulation process as generally used in SISO transmission with a single transmitting antenna and single receiving antenna. The signal separating and extracting means for MIMO realizes signal separation by transmitting a pilot signal sequence made of a number of symbols forming orthogonal patterns to estimate parameters representing a MIMO channel model, and then subjecting the received signals to inverse transform signal processing corresponding to the multiplexing that arose through the MIMO transmission channel. Particularly, the system described in [PTL 1], in addition to spatial multiplexing by MIMO, also uses polarization multiplexing, further including an interference compensator for removing interfering components between polarizations to perform MIMO demodulation on the signal with interfering components between polarizations removed.
Further, Ingson et al. discloses a MIMO demodulating method in which a signal separating means for MIMO is integrated with an equalizer for compensating intersymbol interference resulting from fading [NPL 3]. According to this method, it is possible to perform transmission channel estimation based on simple known signals without the necessity of a pilot signal sequence of some known symbols forming orthogonal patterns, and perform demodulation by controlling the signal processing as to MIMO and the tap coefficients of the equalizer for compensating intersymbol interference based on the error signals after the interference removing process.
In the line-of-sight fixed wireless communications using microwaves or millimeter waves, in order to increase the communication capacity, ultra-multilevel transmission using 1024 QAM (Quadrature Amplitude Modulation) or higher-order QAM has been already used. In order to achieve a further higher capacity, the MIMO transmission technology needs to be used together with the ultra-multilevel modulation. However, when MIMO transmission is applied to fixed wireless communication in the microwave band or millimeter wave band, from the requirements on the intervals between transmitting antennas (and the requirements on the intervals between receiving antennas), specifically, because of the necessity of widening the distances between the transmitting antennas and also widening the distances between the receiving antennas, it is impossible to handle the phase noise arising at each antenna in common with that at the others. Accordingly, it is necessary to treat the phase noise associated with each antenna independent from that of the other. However, there is a problem that such independent phase noise remarkably degrades the quality of communication in MIMO transmission. Therefore, in the line-of-sight MIMO communications system, in order to realize further development of high capacity, high quality and low cost configurations, there has been a demand for a MIMO demodulating apparatus and MIMO demodulating method that has a phase noise compensation function and an interference compensation function.
In the aforementioned MIMO demodulating method using the tandem arrangement of the MIMO signal separation processing and SISO (Single-Input Single-Output) demodulation processing, the temporal variation of phase noise independent at each antenna is faster than the variation of the parameters representing the MIMO transmission channel model. Therefore, the pilot signal sequence consisting of some known symbols forming orthogonal patterns needs to be transmitted at a higher frequency so as to follow the temporal variation of phase noise, which significantly limits the original purpose, or increase of communication capacity. Also in the MIMO demodulating method in which a signal separating means for MIMO and an equalizer for compensating intersymbol interference from fading are integrated, it is difficult, if the phase noise at each of the transmitting antennas and receiving antenna is independent from the others, to avoid degradation of communication quality resulting from this.
As a technology of compensating phase noise in a MIMO communications system, there is a proposal that when MIMO communication is performed between two wireless stations (sites) performing bidirectional communication, measurement of a phase shift is performed based on phase measurement on the forward channel so as to calculate a phase correction to the backward channel based on the measured phase shift and apply the calculated phase correction to the backward channel [PTL 4]. However, since the line-of-sight system is not always applied to bidirectional communication, the method disclosed in [PTL 4] is less versatile. As a technology of compensating phase noise by a process performed on the receiving side, there is a proposal that, in order to compensate phase noise at local oscillation at the receiver, the phase noise at local oscillation is estimated based on a channel estimate determined from the received pilot signal and a transmit replica generated from the signal after error correction, to thereby perform phase noise compensation by multiplying the received signal by the complex conjugate of the estimated phase noise [PTL 5]. However, the technology of [PTL 5] can compensate for the phase noise at local oscillation, but cannot compensate for phase noise that varies independently at multiple receiving antennas.
Further, in line-of-sight fixed wireless communication using the microwave band or millimeter wave band, ultra-multilevel transmission is used together with polarization multiplexing transmission using horizontal polarization and vertical polarization, hence the MIMO demodulating method in the line-of-sight fixed wireless communications system need to be used in combination with such ultra-multilevel modulation and polarization multiplexing transmission. Similarly, also in this case, deterioration of communication quality due to phase noise independently arising at every antenna and for each polarization causes serious problem, so that the number of multiple levels of the signal for each transmission path is limited, making it difficult to use line-of-sight MIMO, polarization multiplexing and ultra-multilevel transmission in combination.